1. Field of the Invention
The present invention relates to a silica glass optical fiber. More specifically, it relates to a radiation resistant single-mode optical fiber which has a low transmission loss even in an environment where it is exposed to radiation, and which experiences a rapid recovery from transmission loss when placed in a radiation-free environment following exposure to a radiation environment.
2. Related Art
When silica glass optical fibers are used in a radiation environment, a loss in the amount of transmitted light (induced loss) arises due to the radiation environment. Optical absorption in the ultraviolet and visible light regions is known to arise due to dopants such as germanium, which are generally used to control the refractive index, and due to impurities present in the optical fiber manufacturing process. For this reason, the wavelength band used for optical fibers employed in a radiation environment has until now been a wavelength of 0.85 μm.
It has hitherto been known that, in the core, pure silica core fibers which are free of germanium and fluorine-doped silica core optical fibers have excellent radiation resistance. In addition, optical fibers of which cores are doped with hydroxyl groups, germanium or phosphorus to further improve the radiation resistance over that of pure silica core fibers have been reported.
For example, “Radiation Resistance of Fluorine-Doped Silica Core Fibers” (Fujikura Giho, No. 86 (1994)) describes the radiation resistance of fluorine-doped silica core optical fibers which are large-diameter optical fibers having a core diameter of 200 μm and a cladding diameter of 250 μm.
Japanese Unexamined Patent Application, First Publication No. S58-125635 discloses a radiation resistant single-mode optical fiber composed of high-purity silica glass to which from 0.1 to several percent of hydroxyl groups have been added. Although it had previously been known that optical fibers with a silica glass core have an excellent radiation resistance, the discovery was made that increases in loss can be suppressed by adding hydroxyl groups.
In addition, Japanese Unexamined Patent Application, First Publication No. H3-247536 describes the results of tests conducted on completely fluorine-doped optical fibers composed of a core and cladding which are both doped with fluorine, wherein the core has a fluorine concentration of up to 0.1 atom % and has also been doped with up to 0.1 mol % of GeO2 or P2O5.
Of the optical fibers used in radiation environments, there is an increased desire for optical fibers which can be employed at the 1.3 μm and 1.55 μm bands normally used for transmission.
However, the optical fibers manufactured in the above Fujikura Giho article were all large-diameter optical fibers having a core diameter of 200 μm and a cladding diameter of 250 μm, and the light source used for evaluating the fibers had a wavelength of 0.85 μm. Hence, no implications are made therein, nor evaluations provided, which serve to indicate what fluorine concentrations and fiber constructions would be desirable for radiation resistant single-mode optical fibers capable of being used at the 1.3 μm and 1.55 μm bands.
In Japanese Unexamined Patent Application, First Publication No. S58-125635, adding hydroxyl groups increases absorption at 1.38 μm, making use difficult in the communication wavelength bands at 1.3 μm and 1.55 μm. Although the wavelength at which transmission loss was evaluated in Japanese Unexamined Patent Application, First Publication No. S58-125635 is not specifically mentioned, the 0.85 μm wavelength appears to have been used.
In Japanese Unexamined Patent Application, First Publication No. H3-247536, the fluorine concentration in the core of the completely fluorine-doped optical fiber is 0.1 atom % or less, which is very low, and the relative refractive index difference with respect to pure silica glass (Δ−) is 0.03% or less. A fluorine-doped glass having a relative refractive index difference with pure silica glass (Δ−) of about 0.07% is also mentioned, but nothing is indicated concerning its radiation resistance. Hence, at fluorine concentrations of the level indicated here, the radiation-induced loss deterioration-suppressing effects are very small.